The present invention relates to a new and improved core material and a method of forming cores to be used in investment casting of an article, such as an airfoil.
When the investment casting of an airfoil is to be undertaken, a wax pattern is formed around a core. To form a wax pattern, the core is placed in a pattern mold cavity. The pattern mold cavity has a configuration which corresponds to the configuration of the airfoil to be cast. Wax is then injected into the pattern mold cavity. This wax surrounds the core to form the wax pattern.
After the pattern has been removed from the mold, the pattern is covered with ceramic mold material. After the ceramic mold material has been at least partially set, the wax material is removed by heating the mold. This leaves a mold cavity having a configuration corresponding to the configuration of the airfoil to be cast. The ceramic mold material holds the core in a position in the mold cavity corresponding to the desired location of internal passages in the airfoil to be cast.
Molten metal is then poured into the mold cavity. The molten metal solidifies to form the airfoil. After the molten metal solidifies, the airfoil is removed from the mold and the core material is removed from the inside of the airfoil. This leaves passages inside the airfoil to conduct cooling fluid flow.
To form a core, a green core is made by injecting a slurry of core material into a core mold. The green core is then removed from the core mold and subjected to two firings. After the first firing, the core is coated with a ceramic binder and is then subjected to a second firing.
In order to minimize breakage, the core must be relatively strong after the first firing. In order to form airfoil passages with smooth inside surfaces, the core must have a smooth outer side surface. In addition, the core must be made of an inert material which does not react with the nickel-chrome superalloys from which airfoils are commonly formed.
Cores for use in investment casting have previously been formed from a slurry containing fused silica, zircon and a binder. This slurry must be injected into very small spaces in a core mold. The small spaces in the core mold are required in order to enable the core to form small passages in an airfoil. The small spaces in the mold must be completely filled with the slurry of core material in order to provide a core having a desired configuration. Therefore, the slurry of core material must have a high degree of flowability. However, the amount of liquid constituents in the slurry must be limited so that the core will have a desired density and strength.
During the firing of the core materials, liquid components in the slurry are driven off. This results in shrinkage of the core from the size to which it is formed in the core mold. This shrinkage must be controlled and accurately predicted in order to maintain the required core dimensional tolerances. Thus, core tolerance ranges on the order of .+-.0.005 of an inch over a length of five inches are necessary for certain cores. The achieving of this accuracy requires the accurate control of shrinkage during firing of the core.